Nothing fancy or exotic here. Just pre-compose and re-use your masks or a duplicate comp as a matte.
Thanks for the response, Mylenium. You'll have to bear with me. I find the complexity of AE intimidating. The morph is working via the PS mask, but I want to learn how I should have done it.
I take it you mean that I should do in AE what I did in PS – apply Mask 1 to the image so that everything outside Mask 1 is black, then precompose, then do the Reshape. All I have to do now is work out how to mask an image in AE. I think I can manage that.
However, what I was asking is: can the moving outline of the Reshape effect itself be applied as a mask? i.e. I want to say to AE: "Hey, you're generating a moving outline, courtesy of that Reshape effect I applied. Now, listen carefully AE, grab that moving outline and use it as a mask on a totally different image".
No. Pixels are pixels and AE is not "aware" of contours outside some effects. I'm not sure why it should be relevant, anyway. The Reshape effect only handles exactly two static masks - one for the end shape, another for the start. Any advanced morphing beyond that requires commercial third-party plug-ins like RevisionFX' RE:Flex or BCC Morph, but then the whole approach would be different, since they work differently. For your specific setup there's per se nothing wrong with doing prep work in PS, though transparencies tend to degenerate with distortion effects and may cause all sorts of gap issues and soft edges. Using AE native masks would eliminate some of these problems due to how the calculations work, but it's a minor thing. The rest will have to be animated manually one way or the otehr based on what I said.
You may not be aware of this, but you don't have to use the reshape effect. On a solid, create a mask. Create a Mask Path keyframe. Move the timeline cursor later in the comp, and rearrange the mask path vertices, which automatically creates another mask path keyframe. Boom -- instant mask animation.
If you're trying to use AE intuitively because of its similarities to Photoshop, I recommend that you don't. There are plenty of "getting started" tutorials right on this website to learn the AE Basics, and your prior knowledge will allow you to progress quickly. But at the same time, you'll probably think to yourself, "Boy, did I have THAT wrong!" as you progress through them.
It's tough to do much of anything in AE without a knowledge of the basics, even with a good knowledge of other applications. There are too many quirks in how AE works that can bite you.